Clay is a sorptive mineral characterized by a layered lattice crystal structure. Clay minerals are derived from condensed forms of silicic acid, H.sub.4 SiO.sub.4, where each silicon atom is surrounded by four oxygen atoms in a tetrahedral structure. These silicate tetrahedra are linked together in regular arrays by the sharing of common oxygen atoms to form chains or two-dimensional sheets and layers.
Metal hydroxides, notably those of magnesium, aluminum, and iron, may also condense to form two-dimensional octahedral structures which can be interdispersed with the above silicate lattices. Various degrees of mismatch and distortion between the tetrahedral and octahedral layers can occur, conferring diverse morphological and chemical properties to the clay. Other factors which influence the character of the clay are the geographical location of the site, and the unique combinations of metals therein.
In the crystalline lattice, the irregular series of layers with corresponding interstitial spaces comprise pores which avail the clay mineral to adsorption phenomena. The metal ions bound within the lattices may also play a role in the solvation or hydration of clay in light of the mechanisms of ion exchange and hydrogen bonding. Osmosis can be a factor in adsorption as well. Polarized water molecules adsorbed onto the surface of mineral clays present an important property of many clays, namely their surface acidity. In fact, the surface acidity of dried mineral clays may exceed that of concentrated sulfuric acid. This can result in the catalysis of many reactions, desirable or otherwise.
Pesticides, including many insecticides, fungicides, nematocides, and herbicides, are often prepared as dry formulations that contain the pesticide impregnated onto various organic and inorganic carriers in their granulated or powdered forms. Collectively, these pesticides are often referred to as "active ingredients" and are represented by such diverse chemical classes as organophosphate esters, carbamates, pyrethroids, polyhalogenated hydrocarbons, or the like. Examples of organic carriers are corn cob grit, peanut hulls, and pecan hulls. Examples of inorganic carriers are clay minerals such as montmorillonite, attapulgite, sepiolite, vermiculite, kaolinite and various other mineral ores, e.g., gypsum, diatomaceous earth, and the like. Sand can be employed as a carrier in certain circumstances. The decision to employ a preparation composed of such pesticide/carrier mixtures is dictated by the safety, efficacy, ease of application, and nature of the target.
Generally speaking, it is desirable that carrier materials used for granular pesticide preparations possess sufficient hardness such that the formulated granules are not easily reduced to powders and possess enough porosity so that high loadings of the active ingredient can be achieved. Hardness is particularly important from the standpoint of worker exposure. If the formulated granules can be easily reduced to fine powder/dust by handling, the dust can become airborne and inhaled by workers. Porosity or "liquid holding capacity" (LHC) of the granular material refers to the ability to retain pesticides internally, thereby keeping the surface of the granule dry. It is desirable that granules have high LHC, which reduced the total mass of preparation that needs to be applied and thereby increases per pound efficacy.
Another important property of granular products destined for use as carriers for pesticides is inertness. Inertness refers to the lack of chemical reactivity with an active ingredient such as a pesticide. It is well known in the industry that some carriers possess sufficient chemical reactivity that they are capable of degrading or partially decomposing active ingredients.
For example, the common pesticides parathion and malathion are particularly susceptible to hydrolysis when absorbed onto the surface of most aluminosilicate clay minerals [U. Mingelgrin, S. Yariv, and S. Saltzmann, J. Soil Sci. Amer., 42, 519 (1978)]. It has been reported that the exchangeable metal cations in mineral clays are potent catalysts for the hydrolysis of pesticides [M. M. Mortland and K. V. Raman, J. Agr. Food Chem., 15, 163, (1967)]. Other agricultural chemicals which are susceptible to hydrolysis on clay mineral surfaces include the chlorotriazine herbicides as well as the chlorinated aromatic and alicyclic pesticides.
From the standpoint of the formulator using granular carriers, pesticide stability is of concern for both regulatory and economic reasons. Pesticide preparations used as items of commerce are carefully monitored by both state and federal agencies which regulate the level of the active ingredient claimed on the label plus allowed variance. The formulator may be required to pay substantial fines and/or remove the product from the marketplace if the product is in violation of the regulation. With regard to economics, since the pesticide is the active ingredient, any loss of its activity due to degradation reduces efficacy, thereby making the product less competitive or cost effective in the marketplace. Also, since the active ingredient is the most expensive ingredient in these preparations, the formulator desires to use the smallest amount of pesticide while still meeting the primary objective of preparing an efficacious product. If "extra" pesticide in the preparation is required to make up for degradation losses due to lack granule inertness, this adds cost and again makes the product less competitive in the marketplace.
Pesticide formulators are well aware of the pesticide stability issue and there are numerous patents which claim solutions to this problem. The most common approach involves using an additional ingredient in the preparation to moderate or reduce the chemical activity of the carrier toward the active ingredient. Sometimes called "deactivators", these ingredients are generally organic molecules such as organic amines, lactones, organic acids, glycols, alcohols, and trialkylphosphates which can be used alone or in combination. Usually they are used at levels in the 1-4 wt. % range, but sometimes higher levels are required. Although such deactivators generally cost from $0.60-$1.83 per pound and can add from $50-$75/ton to the cost of finished goods, the added expense is justified by having an active product.
It is also understood that some types of clays, particularly attapulgite, can be made more stable (i.e.--less chemically reactive) simply by heating at elevated temperatures. For example, heating either kaolin or attapulgite to 1000.degree. C. (1832.degree. F.) reduces the surface area of the carrier [B. Valange, J. Marcoen, J. Closset, Parasitica, 31, 135 (1975).] Heat-treated carriers exhibit a decreased tendency to degrade the pesticide malathion. U.S. Pat. No. 3,232,831 to Schwin describes thermal treatment in the range 1500-2000.degree. F. of attapulgite, such that the surface area of the clay is reduced to below 20 m.sup.2 /gm. Such a carrier is highly stable for the organophosphate ester Diazinon.TM.. U.S. Pat. No. 3,278,040 to Goldberg et. al. describes a method of making better filter aids consisting of fluxing attapulgite with various salts including lime and feldspar with sodium or calcium carbonates.
While high temperature thermal processing is simple, there are negative performance and economic consequences associated with such an approach. In particular, the extra fuel required to achieve such high temperatures substantially increases the cost of finished goods on a per ton basis. Furthermore, the liquid holding capacity of attapulgite clay is substantially reduced when subjected to high temperature thermal processing, thus limiting the amount of active ingredient which may be added.
The present invention describes a fluxation process for manufacturing clay-based materials that exhibit substantially improved stability characteristics as carriers for labile or sensitive pesticides. In addition, such improved carriers require less organic deactivator to be used for stabilization. The present process of fluxation includes heating salt-impregnated precursor clays under conditions sufficient to obtain clay-based products possessing enhanced inertness and thus enhanced product stability.